This invention relates generally to apparatus and methods for sensing changes in physical phenomena. More particularly this invention relates to apparatus and methods for sensing changes in acoustic fields. Still more particularly, this invention relates to fiber optic apparatus and methods for sensing changes in acoustic fields.
A gradient hydrophone produces an output that is directly proportional to the gradient of the acoustic pressure in water. Previous gradient and direction finding hydrophones are electro-mechanical and typically use sensors that include piezoelectric, electromagnetic, or capacitive transduction elements. Some of these devices are configured as neutrally buoyant bodies having piezoelectric accelerometers or moving coil, velocity-type pickups mounted within relatively rigid cases.
All previous gradient hydrophones are electro-mechanical devices and as such are subject to electromagnetic pickup, which causes noise. When several electro-mechanical hydrophones are used in arrays, electrical crosstalk and other sources of electrical noise, such as ground loops, frequently cause problems.
Piezoelectric devices have relatively high impedances and are subject to capacitive loading; thus preamplifiers must be installed as close as possible to the device to prevent sensitivity degradation. These preamplifiers require delivery of electrical power thereto, which further adds to the cost and complexity of such systems.
A typical electromechanical hydrophone system includes twisted pair cables or coaxial cables as output leads. These output leads limit the bandwidth of the hydrophone systems. In addition, the cables required for arrays of hydrophones are heavy, large in cross section and expensive. Furthermore, in applications of current interest, size and weight restrictions greatly limit the useable number of hydrophone elements and reduce the effectiveness of the systems.
An optical fiber comprises a central core and a surrounding cladding. The refractive index of the core is greater than that of the cladding. Light is guided by the core if it impinges upon the core-cladding interface at an angle less than the critical angle for total internal reflection.
A light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate withous severe attenuation. The waves having field components that propagate unattenuated are called normal modes. A single mode fiber will propagate only one spatial distribution of energy, that is, one normal mode, for light of a given frequency.
Optical fibers are sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber exposed to such phenomena changes the amplitude, phase or polarization of light guided by the fiber. Optical fibers have been considered for use as sensing elements in devices such as microphones, hydrophones, magnetometers, accelerometers and electric current sensors.
Optical fiber elements configured as Mach-Zehnder, Michelson, Sagnac, and resonant ring interferometers have been used as sensors. Interferometers respond to the phenomenon being sensed by producing phase differences in interfering light waves. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored.
A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. The sensing arm is exposed to the physical parameter to be measured, such as an acoustic wavefront, while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave pressure amplitude. An optical coupler divides a light signal between the two arms. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.